In radars providing a visual presentation of the returns from the illuminated terrain, such as side looking radars, the beam is divided longitudinally into a plurality of range bins, each of which represents a specific amount of travel of the radar signal in a longitudinal direction. As a specific example, each bin in the present disclosure represents 30 meters of ground travel of the radar signal. The radar visual display is a cathode ray tube (CRT) on which the display is actually composed of a plurality of pixels. Each pixel is generally considered to be rectangular and the dimensions, height and width, are referred to as the aspect ratio. For example, if the aspect ratio is 1 to 1 and a range bin is 30 meters long the pixel will represent an area 30 meters by 30 meters. However, for an aspect ratio of 4 to 3, wherein the range bins are 30 meters, the azimuth dimension of the pixel will be 22.5 meters. Also, the aspect ratio of the pixels determines the overall dimensions of the display.
Radars are normally pulsed many times per second and the interval between pulses is the time during which return signals are received. This pulse repetition interval (PRI) may be changed during operation of the radar for various reasons. When the PRI of the radar is changed or when the ground speed of the airplane carrying the radar is changed, the amount of terrain covered in each range bin in the azimuth direction, will vary. The returns in each range bin are integrated for a number of PRI to provide the video data for each pixel. Also, the system usually is a dual system alternately covering the terrain on both sides of the airplane.
In the prior art, the radar beam is divided into transversly extending azimuth cells with the transverse dimension of the azimuth cell being determined by a fixed number of PRI. Since the radar operator may vary the number of PRI per unit of time and since the ground speed of the airplane carrying the radar may vary, the transverse distance of the azimuth cells will vary substantially, which will radically alter the aspect ratio of the pixels and the visual display. These radical changes in the azimuth cell size make it very difficult, if not impossible, to scale the output for display on a raster scan CRT.